The present invention relates to a method of machining a press die for press working.
The press die comprises an upper die, a lower die, and a guide unit for guiding the dies. The press die is roughly machined to form various contours (shapes), so that there remain many cutter marks which are so called cusps. The cusps are ground and then the surfaces are coated with a wear resistant material to enhance wear resistance as described in Japanese Utility Model (unexamined) Publication No. 63 (1988)-138924.
The cusps are removed to smooth whole surfaces of the press die by grinding, and hence a grinding operation requires many manhours. Moreover, it is hard to obtain accurate matching of the upper and lower dies with a very small clearance in the whole area. If the accurate matching is insufficient, a sheet metal is locally and strongly pressed, and therefore an excessively large clearance can be formed. When the work is locally and strongly pressed, an excessively large load is applied to a press unit, and a shape of the final product does not become sharp since the press working is not performed at a lower dead point. When the clearance at a portion of a work is excessively large, the work does not match with the surfaces of the press die. And a desired shape of the product cannot be produced. In addition, the accuracy of the products may change lot by lot.